The Water Cycle

The Water Cycle

Water cycle diagrams often summarize the water cycle in a qualitative way, noting reservoirs and flows. Reservoirs are places where water is "stored", or where it stays for some period of time. The oceans, glaciers and ice caps, lakes, and the atmosphere are some examples of reservoirs. Flows or pathways are the routes water takes between reservoirs. Evaporation moves water from the oceans to the atmosphere. Precipitation moves liquid (rain) or solid (snow) water from the sky back to Earth's surface. Snowmelt runoff turns solid water into a liquid that flows down rivers to the sea. The amount of time that water spends, on average, in a reservoir is called reservoir time.

Note that "water", in this context, means the chemical substance H2O, whether in liquid, solid, or gaseous form. Water in oceans and lakes is, of course liquid; but it is solid ice in glaciers, and gaseous water vapor in the atmosphere.

Water cycle

How much water is there in the water cycle?

A quantitative look at the water cycle adds context to a water cycle diagram. For example, the oceans are by far the largest reservoir, containing between 1.35 and 1.4 x 109 km3 of water; this is more than 95% of the water in the cycle. (See table 1)

And how much of it is moving from place to place?

The rate of a flow, commonly expressed as cubic kilometers per year, can be measured worldwide. For example, total global precipitation is about 505,000 km3 per year. (See table 2)

How long does water spend in one place or another?

Reservoir time is highly variable, depending on the type of reservoir. For example, water stays in the atmosphere for only nine days on average. By contrast, once water reaches the ocean, it stays put for a very long time - an average of more than 3,000 years. It is important to realize that reported residence times are averages, and that the actual residence time for a given water molecule may be far from the average. Water vapor that reaches the stratosphere may remain there for a long time; water that flows into warm, shallow coastal waters from a river may evaporate and leave the ocean very quickly.  

A few highlights of where water spends its time are below. See table 3 for more detail.

  • Atmosphere: The residence time for water in the atmosphere is a brief nine days; the shortest of any major reservoir. 
  • Soil moisture: Residence times for moisture in the soil average around one to two months. Roughly 122,000 km3 of water is contained in soil reservoirs. Transpiration puts 21,000 km3 of water into the air (essentially the same amount as is absorbed from the soil by plants), while evaporation from land accounts for 50,000 km3 of water in the atmosphere.
  • Ice Sheets: Water spends thousands, tens of thousands, or even hundreds of thousands of years in a frozen state in an ice sheet before returning to other parts of the water cycle.  (For example, the oldest ice found in Antarctica is about 900,000 years old.)
  • Glaciers: Water resides in glaciers for less time than it does in ice sheets, often about 20 -100 years. 
  • Snow: Water spends around 2 to 6 months as seasonal snow.  Each year approximately 11,000 km3 of water is in snow; about the same amount melts into water.
Table 1: Amounts of water in the cycle
Reservoir Volume % of a larger reservoir
All of Earth's water 1,386,000,000 to 1,460,000,000 km3 NA
Oceans 1,338,000,000 to 1,400,000,000 km3 97% of total water
Fresh water 35,030,000 km3 2.5 to 3% of total water
Ice & snow 43,400,000 km3 -
Ice caps, glaciers, and permanent snow 24,064,000 to 29,000,000 km3 68.7% of fresh water
about 2% of total water
Antarctic ice & snow 29,000,000 km3 about 90% of all ice
Greenland 3,000,000 km3 about 10% of all ice
Mountain Glaciers 100,000 km3 -
Ground water (saline+fresh) 23,400,000 km3 -

Ground water (saline)

- 54% of ground water
Ground water (fresh) 10,530,000 km3 30.1% of fresh water
46% of ground water
Surface water (fresh) - 1% of fresh water
Lakes - 87% of surface fresh water
Swamps - 11% of surface fresh water
Rivers - 2% of surface fresh water
Atmosphere 12,000 to 15,000 km3 -
Table 2: Flows between reservoirs
Process From/to Reservoir >Flow Rate
Precipitation Atmosphere to Ocean/Land 505,000 km3/year
Ocean precipitation Atmosphere to Ocean 398,000 km3/year
Land precipitation (except snow?) Atmosphere to Land/surface 96,000 to 107,000 km3/year
Evapotranspiration Ocean and Land/surface and Plants to Atmosphere 505,000 km3/year
Ocean evaporation Ocean to Atmosphere 434,000 km3/year
Land evaporation Land/surface to Atmosphere 50,000 km3/year
Transpiration Plants to Atmosphere 21,000 km3/year
Uptake by plants Land/surface to Biota 21,000 km3/year
Runoff Land/surface to Ocean 36,000 km3/year
Melting Ice/snow to Land/surface 11,000 km3/year
Snowfall (on land only?) Atmosphere to Ice/Snow 11,000 km3/year
Percolation Underground to and from (??) Land/surface 100 km3/year
Table 3: Residence times in reservoirs
Reservoir Residence Time (average)
Oceans 3,000 to 3,230 years
Glaciers 20 to 100 years
Seasonal Snow Cover 2 to 6 months
Soil Moisture 1 to 2 months
Groundwater: Shallow 100 to 200 years
Groundwater: Deep 10,000 years

Lakes

50 to 100 years
Rivers 2 to 6 months
Atmosphere 9 days

A little water cycle accounting:

The overall water cycle "contains" between 1,386,000,000 and 1,460,000,000 km3 of water in various states (liquid, solid, or gaseous).  The vast majority of it, between 96.5 and 97.25%, is in the oceans.  Only about 3% of Earth's water is fresh water. About 2/3 of that is frozen in the ice sheets near the poles and in glaciers. About 90% of the polar ice sheet and glacial ice is in Antarctica; most of the rest is in Greenland; a tiny fraction is locked up in mountain glaciers elsewhere. Most of the rest of the fresh water is underground. Less than 1% of fresh water is on the surface in lakes, wetlands, and rivers.

About 80% of rain falls directly into the ocean. However, more water leaves the ocean via evaporation (434,000 cubic km of water per year) than enters the ocean as precipitation. The opposite is true on land: less water evaporates over the land (71,000 cubic km per year) than falls onto land as precipitation. Evaporation of water from the land happens directly from lakes, puddles, and other surface water. And water also makes its way into the atmosphere via plants and a process called transpiration. Collectively, the water evaporated from the land and from plants is called evapotranspiration.

Higher rates of evaporation over the ocean and more precipitation over the land would present an imbalance if it were not for river runoff that transports water from land to ocean. The accounting only works out if the amount of runoff that flows in rivers to the ocean makes up for most of this imbalance.

  • 434,000 cubic km of water evaporates from the oceans each year, while 71,000 km3 (about 1/6th as much) rises into the air over land via evaporation and transpiration.  
  • About 398,000 km3 of precipitation falls upon the oceans each year, while 107,000 km3 (96,000 km3 of rain and 11,000 km3 of snow) falls upon land. 

A note about numbers:

Some of the quantities listed below are ranges. This is partly because there is some uncertainty and variability in the exact values for some quantities; and partly because different data sources group reservoirs and flows slightly differently. For example, snow might be considered a component of the "surface water" reservoir, or it might be a component of the "snow and ice" reservoir. Some estimates report a combined amount of evapotranspiration, while others separate ocean evaporation, land evaporation, and transpiration. You may, therefore, find slightly different numbers in other data sources.

How is the water cycle affected by climate?

Which reservoirs and flows are most likely to be key players in the climate change story? Which quantities, in terms of reservoir sizes, flow rates, and residence times, are most important to have in mind when considering climate change issues? The readings that follow provide an overview of a few key climate change issues that involve the water cycle.

© 2011